It's hard to say where the lower bound for the value of American companies is now. Is it zero?
Usually one could estimate the absolute minimum of future earnings.
But the current administration seems to be set on redirecting those earnings away from investors towards the government. And I don't see what could limit the extent to which they do so.
The companies will continue to produce valuable stuff. But will they make a profit? Is an Apple that gives billions of dollars to investors (many of them abroad) better for Trump than an Apple that gives those billions of dollars to the government which he controls?
Tariffs are one way to redirect those profits. With their margins of over 30%, Apple can probably keep selling hardware at the same price, even when they have to spend more on the components. Everything might stay the same, except what previously was profit might now go to the government via tariffs or other means. Making US companies worthless to investors.